Conventionally, spot welding has widely been utilized as one of the methods for welding a metal member. In spot welding, a welding gun, in which a cap tip is fitted to a leading end of a shank, is often attached to a robot arm. However, if the welding gun is used for a long time, a shape of the leading end of the cap tip becomes deformed and worn, and it is hard to secure a quality weld. Accordingly, the shape of the leading end of the cap tip is maintained by automatically grinding the cap tip periodically after a fixed processing time or a fixed number of welding points, or by replacing the cap tip from the leading end of the shank with a new or ground one.
On the other hand, in recent years, because a rust proofing process using a chemical treatment agent is frequently applied to a welded material, the cap tip wears out early requiring more frequent replacement. However, stopping a production line and replacing the cap tip lowers production efficiency. Accordingly, in order to make it possible to replace the cap tip without stopping the production line, there is proposed an automatic detaching apparatus of a cap tip that is attached to a leading end of a robot arm, for example, as shown in Japanese Unexamined Patent Publication No. 2002-79382.
However, the conventional automatic detaching apparatus of the cap tip described in this publication is structured such that one fork-like lever is inserted into a gap between the cap tip and the shank, the lever is inclined in a state of being applied to the shank side, and the cap tip is detached in such a manner as to be unclenched in through an application of leverage. In this case, there is a problem that a concentrated load is applied to a corner portion of the shank and the tip with which the lever is brought into contact causing a scratch. Further, there is another problem that the lever is not inserted properly to the gap between the cap tip and the shank, and the cap tip fails to be detached.